1. Field of the Invention
The present invention relates to electrochemical cell components, and more specifically to separator plates within electrochemical cell stacks.
2. Description of Related Art
Electrochemical cells having proton exchange membranes (PEM) may be configured into electrochemical cell stacks. Each of the electrochemical cells in the stack are separated from adjacent cells with bipolar separator plates. These bipolar separator plates are typically made from a variety of metals, such as titanium and stainless steel, and from non-metallic conductors, such as graphitic carbon. Bipolar separator plates may be fabricated by machining fluid flow fields into solid sheets of material. The flow fields typically comprise a series of channels or grooves that direct the flow of gases and liquids over the faces of the electrodes.
In a typical electrochemical cell, the membrane and electrode assembly (MEA) may be supported on either side by the fluid flow fields and the bipolar plate, and are surrounded by cell frames that provide sealing surfaces and support. The cell frames have at least one manifold or flow passage, and often have several manifolds to direct the reactant and product fluid streams to their respective cathode or anode sides of the electrochemical cells. In a single cell arrangement, cell frames, flow fields and bipolar plates are provided on each of the anode and cathode sides.
The cell frames are usually sealed by means of sealing ridges, which are embossed, machined, or molded into the frame. The sealing features react against gaskets included in the stack to maintain fluid tight joints and also to grip the gaskets, which prevents creep and extrusion of the membrane. The electrochemical cell stack is conventionally placed under compressive forces by the bars that are threaded though the stack and apply compressive forces through the end plates at each end of the stack. The compression of the fuel cell stack applies the sealing force to the sealing surfaces between the separator plates and membranes. Such seals typically circumscribe the manifolds and the electrochemically active area on the cell frame.
In addition to providing a fluid flow field and structural support for the MEA, a bipolar separator plate also collects electrons liberated at one electrode, conducts the electrons through the plate, and delivers electrons to the face of another electrode on the opposing side of the plate. In this manner, the bipolar plate acts as a current collector and therefore is usually constructed of a conductive material.
Conventional PEM electrochemical cells may have bipolar plates that are constructed from a variety of materials of construction depending on the service of the electrochemical cell stack and the corrosiveness of the reactants and products flowing through the stack. Often the bipolar plate may be fabricated from a solid sheet of material having the flow fields machined or stamped into the surface of the solid sheet. Alternatively, the bipolar plates may be formed in molds having flow fields included in the mold design.
Watkins, et al., in U.S. Pat. No. 5,108,849, issued Apr. 28, 1992, discloses a fluid flow field plate made of a conducting material, preferably graphite. A fluid flow channel is machined, stamped or molded into the surface of the fluid flow field-plate. The disclosed fluid flow plate has a fairly high density and lends considerably to the overall weight of an electrochemical cell stack.
Mercuri, et al., in U.S. Pat. No. 6,037,074, issued Mar. 14, 2000, discloses a fluid flow field plate constructed of a graphite sheet impregnated with a resin and dispersed throughout with needle shaped ceramic fibers. A flow channel is stamped into one surface of the graphite sheet and the opposite surface is backed with a second graphite sheet to impart rigidity to the assembly.
It is apparent that the bipolar plates and cell frames contribute greatly to the bulk and weight of electrochemical cell stacks. Accordingly, there remains a need for bipolar plates having significantly less density so that the overall weight of the electrochemical cell stack may be reduced. It would be an advantage if the bipolar plate also had reduced dimensions, thereby further decreasing the weight of the electrochemical cell stack.